Spiral wound gasket keeps unwinding shortly after startup

[lostmeche]

Hello.

We have a 300PSIG MAWP - 20700lb/hr natural gas powered boiler. We have had three different spiral gaskets unwind shortly after startup. Two of them are installed by one tech and the third by different tech. The first one started leaking right off the bat where as the second one unwound but did not leak for several weeks. The third one was installed yesterday and unwound shortly after startup. It is the gasket for the man hole cover. The cover is held in place with two studs and it is an oval shaped gasket.

Could we have perhaps gotten a bad batch of gaskets? Since it's two different techs that installed them and it's only two studs to tighten, I don't believe installation is the issue. I'm not 100% sure as I have not seen them put them in. We have a contractor do our boiler work. Should I ask to use a solid graphite gasket instead?

 

[SnTMan]

lostmeche, I am not familiar with the design details for your application, but for most classes of work it is preferred to have a solid ring, sometimes called a compression ring* at the SW gasket OD and ID. This prevents the unwinding you are seeing. Don't know if your manhole cover dimensionally permits it or not. Is this an oval form?

Not sure I'd go with solid graphite, these can sort of delaminate in service. A metal insert or something of the sort is needed. Maybe a corrugated metal, graphite faced gasket can be used. I'd contact a reputable gasket manufacturer.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

EDIT: * AKA centering ring

 

[TGS4]

What's the size that there is only two studs?

 

[mk3223]

Can be shared either a sketch or photo of the nozzle flange type and SW gasket used?
Also, consider to involve the gasket vendor to investigate the incident together. The vendor SME may have a solution ready for your case.

 

[SnTMan]

Maybe

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[BJI]

Under normal operation the internal pressure is sufficient to keep the manway sealed, so the two bolts are only needed to keep the gasket seated at start-up. Sometimes the bolts are adjacent to the gasket edge, not more centralized like the prefabricated manways shown in the link supplied, which could be the main issue. Spiral wound gaskets are more sensitive to poor assembly and can be locally damaged if the bolt loading is not relatively uniform over the perimeter. Under operating conditions you likely don't need inner and outer rings but they may be sufficient to prevent the windings from extruding. The gasket may still be damaged even with the rings, so I would change to a graphite covered corrugated or kammprofile, as suggested by SnTMan. It would be safe to say you also need to reduce your gasket area and back off the assembly bolt load.

 

[DriveMeNuts]

So the Manway is sealed by the door pushing onto the gasket under internal pressure and the two bolts are just positioning/retaining bolts?

I don't see how two bolts could compress an entire manway sized gasket. Something doesn't make sense here.

 

[racookpe1978]

Unwinding, as pointed out above, is best solved by buying a gasket with an inner and outer solid ring. That's the only type we are allowed to purchase, so I've never seen unwound gaskets in any steam system.

Two bolts? Seems too few for a manhole, but likely for a small hand hole or inspection cover for a steam system that has a clamping bracket inside. Common on older boilers and superheater inspection points.

Machining. Have you verified the two mating surfaces are flat, square, non-gouged, parallel to each other when closed? Are the two clamping "arms" (brackets) true and parallel, with correct seating surfaces to the metal around the inner wall of the boiler/HRSG flange?

Mechanically, look at how the man hole/doorway/hatch physically moves and slides and is twisted as it goes in and out the hole. Could be hitting and damaging the gasket physically during each re-installation of the door/hatch/hand hole cover. A bad pivot point could be twisting the cover as it seats the last 1/16 of an inch.

 

[lostmeche]

@Sntman: The gasket is in oval form. I'm not entirely sure if a gasket with an outer and inner ring is possible. And I'm not sure if that is what is being used. Every time they come to install them I'm tied up somewhere else. I'm waiting to hear back from the contractor what gasket type, brand they are using. Thanks for your warning about solid graphite.

Everyone else: The gasket is a 12x16 oval gasket. I'm not sure what thickness or specifically which type of spiral wound gasket is being used. I did think about poor installation but I have a hard time believing they would mess it up with only two studs. I will request that they have me present next time so that I can inspect the sealing surfaces as well as their installation method.

I have also attached a picture of the cover.

 

[SnTMan]

lostmeche, "solid graphite" is usually a bunch of thin lamella, not very well adhered to one another. They can slide past each other and out of the joint. Guess how I know this.

DriveMeNuts, buildings have doors. Pressure vessels have covers

The problem with sloppy work is that the supply FAR EXCEEDS the demand